Flow molding with the use of high frequency energy such as radio frequency energy has come into widespread use in recent times particularly in the surface molding of shoe uppers. As well-known, in such procedures, a negative RTV silicone rubber mole is produced, positioned on a ground electrode and a sheet stock such as vinyl sheet stock to be surfaced is laid over the mold with a top electrode pressed thereon and radio frequency energy flowed through the mold to thermally soften the vinyl and allow it to take on the surface characteristics of the negative mold.
The molds used are frequently made from RTV silicone rubber in known flow mold making procedures. However, such RTV silicone rubber mold masters have limited life spans in high frequency flow molding. The RTV silicone rubber molds act to absorb heat causing increased dwell times in the mold and in some cases, absorb plasticizers and secondary plasticizers of the vinyl thereby causing weakening, swelling and dimensional change of the silicone rubber molds. The definition of the surface configuration to be transferred is sometimes lost after few molding operations. The silicone rubber in some cases tears or permanently distorts. These defects in the silicone rubber molds used in flow molding are well-known.
It has been suggested that more durable molds be formed. For example, epoxy molds have been suggested. However, it is found that when such epoxy molds are used in radio frequency flow molding, the epoxy tends to absorb heat at a rate 3 to 8 times greater than absorbed by silicone rubber thereby lengthening dwell times in the mold to an unacceptable degree. Moreover, detail of the surface configuration to be transferred is sometimes lost. The removal of heat by the epoxy is such that in some cases the vinyl sheet being molded never reaches a molten state to allow detail to be transferred.
Probably because of the foregoing problems, the art has in most cases continued to use RTV silicone mold masters in high frequency flow molding procedures.